Hitherto, for packaging with trays made of various materials, stretch-packaging shrink-packaging, and the like using packaging films has been employed. In the stretch-packaging, since a tray is packaged with stretching the film, the packaging has a characteristic that the film is not loosen and thus a good package appearance can be maintained even after the resulting packaged body is transported. Moreover, in the shrink-packaging, particularly pillow shrink-packaging, an article to be packaged is generally packaged by the following method. Namely, after an article to be packaged, such as a food placed in a tray is loosely packaged with a film with a margin independent of the shape and size of the article to be packaged, center sealing is conducted toward the film on the backside of the packaged body by a rotating roller thermal fusion method, further the film before and after the tray is sealed by a seal-and-cut method, and then heat is applied to the packaged body for an extremely short period in a thermal shrinking tunnel to shrink the film, whereby the article to be packaged is rapidly and tightly packaged. Therefore, in the pillow shrink-packaging, it is possible to package several kinds of trays without changing the width of the film for packaging trays. As a result, the pillow shrink-packaging has characteristics that laborsaving in the packaging step is possible and the packaging speed can be also accelerated as compared with the stretch-packaging.
However, in any of the above packaging methods, the whole content including a tray is packaged. Therefore, in the above packaging method, in the case that rough transportation is performed at the time when a raw meat or the like is packaged, for example, there arises a problem that meat juice may seep into the backside of the tray and hence hygiene is not maintained when the packaged body is turned upside down.
As a method capable of solving problems in such stretch-packaging and shrink-packaging, there is a top-seal packaging wherein a tray and a film is adhered or fused at a circumferential edge part of the upper face of the tray and thereby the tray is sealed. In this method, since the circumferential edge part of the upper face of the tray is surely adhered or fused for sealing, the seeping of the water and oil coming from its content such as a raw meat into the backside of the tray and the leakage of the water and oil do not occur. The quantity of the film to be used is also small as compared with the stretch-packaging or shrink-packaging. Thus, since it is possible to reduce the quantity of packaging material, it is an effective means for waste reduction.
However, in the case that a tray made of a polystyrene-based resin as a raw material is sealed by the top-seal packaging, it is impossible to seal the tray with a commonly employed film using an olefinic resin as a surface layer since the film is not fused to any of a hard tray and a foam tray. Therefore, there is a case that the tray is transformed into a multilayer one or is coated and thus a multilayer tray wherein an olefinic resin layer is arranged on the sealing part of the tray is used. However, in such a tray, since a polyolefin resin is laminated or coated on a polystyrene layer in the production step of the tray, not only the production step becomes vexatious but also loss generated in the production step is difficult to recycle and reuse since the loss is a mixture of two or more kinds of raw materials. There is a method of applying an adhesive to the circumferential edge part of the tray but, as is similar to the case mentioned above, the production step of the tray becomes vexatious and recycle and reuse thereof becomes difficult.
Against these problems, Document 1 discloses a stretch film wherein a layer mainly composed of an ethylene-based resin and/or an ethylene-α-olefin copolymer and a layer composed of at least two polymer blocks mainly made of vinyl aromatic compounds and at least one polymer block mainly made of a conjugated diene are arranged on the surface layer, respectively.
However, the stretch film is insufficient in sealing property in a state that the content is heaped on the tray beyond the height of the tray, particularly intervening material sealing property. After the packaging, when several to several dozen pieces of packaged bodies are packed in a cardboard box in layers, the packaged bodies in lower layer, particularly the packaged bodies in the lowest layer are dented since pressure is applied to the content owing to the weight of the upper layer(s), so that the film surrounding the dented part is loosed. The loosed film results in bad appearance and lowers its merchantability at the store.
[Document 1] JP-A-10-180951